The present invention relates to a process for the manufacture by the float method of glass sheets intended for the production of glazing.
The manufacture of sheet glass by the float method is carried out in a known way by pouring molten glass onto a liquid support (or "bath") which is denser than the glass (for example a tin bath) and by causing the glass strip which is formed to progress over the surface of the bath before cutting the said strip, solidified by cooling, into sheets.
A current process consists in carrying the glass strip over the surface of the bath by seizing it laterally with top rolls positioned along the bath on both sides of the glass strip. Plants which use this process are generally high throughput plants (up to several hundred tonnes of glass per day) and are suitable for the production of glass sheets with a thickness of several millimeters. Such a process exhibits certain disadvantages: in particular, the energy consumed is high (required, inter alia, to cool all the top rolls over the length of the bath), the side edges of the strip marked by the top rolls have to be removed, which involves a not insignificant loss in material due to the size of the top rolls, and a phenomenon of necking (striction) of the glass strip at the outlet of the final top rolls is observed, it being possible for this necking to cause defects, in particular optical defects, on the glass. In addition, the necking observed for such a process increases when the thickness of the sheet produced decreases and can be crippling in the production of thin glass sheets (with a thickness, for example, of less than 3 mm). In the same way, the width of the side bands marked by the top rolls increases when the thickness of the sheet produced decreases due, in particular, to the use of a larger number of top rolls over a greater width of the strip. The geometric yield (proportion of glass remaining after cutting off the marked side bands) is thus particularly low for low glass thicknesses.
According to another process which has formed the subject of Patent FR 1,378,839 and of its additions numbered 86.221, 86.222, 86.817, 87.798 and 91.543, Patent FR 2,123,096 and Patent FR 2,150,249, continuous and flexible moveable guiding elements (for example metal wires), which adhere to the glass over the side edges of the strip and accompanying the said strip in its movement over the bath, are used. This process is more economical than the process using top rolls and the necking observed is also much weaker. However, for low thicknesses, the process as described in the abovementioned patents does not make it possible to obtain a satisfactory quality either, the glass sheets obtained not exhibiting, in particular, a constant thickness (observation of a "hollowed outline" with a thinning of the sheet at the centre).